The present invention relates generally to electrical power instrumentation and more particularly relates to a current and voltage transducer circuit for providing a current source output having an output direct current which is directly proportional to the average and the effective value of a sinusoidal input voltage or current.
Current and voltage transducers are commonly utilized for monitoring and recording currents and voltages at various locations in a power distribution network and are also useful for providing analog input data for use by a power control system. Power systems utilizing such transducers may typically operate at 25 Hz, 50 Hz, 60 Hz, 400 Hz and even up to 1000 Hz.
Various operating characteristics are recognized as being desirable in such transducers. A highly linear transfer function is desired so that the output current accurately represents the magnitude of the sinusoid over a broad range of input magnitudes. It is also desirable that the output current be a smooth direct current. However, although ripple minimization is desired, a fast response time is also desired in order that there be little time delay from changes of the input sinusoid to corresponding output changes. It is also desirable that such transducers operate as current sources and therefore a circuit is needed which has a high output impedance so that the impedance of the load being driven by the transducer has no significant effect on the output current.
Traditionally, the need for a low ripple and a fast response have been met by an engineering trade off. Reducing ripple ordinarily produces a slower response time while a faster response time ordinarily was accompanied by increased ripple. There is therefore a need for a circuit which can provide decreased ripple with a faster response time.
Non-linearities of transducer circuits have been recognized as arising primarily from the non-linear characteristics of iron core transformers and the non-linear characteristics of the finite output impedance of the transformers working into the P-N junctions of the semiconductor devices in the circuit. In the past, these non-linearities have been overcome by using large transformers which provide large voltages and currents so that the relative effect of the non-linear circuit elements upon the circuit operation becomes insignificant compared to the effect of the linear circuit elements such as resistances and capacitances. Thus, in the past, linearity was sought by an attempt to make the gain and transfer function of the circuits determined, to a reasonable approximation, by the values of the linear circuit elements.
However, the use of large transformers and the operation of circuits with relatively large voltages and currents increases circuit cost, heat dissipation, power consumption as well as producing highly non-linear operation near the low magnitude boundary of the operational range of the transducer.